Correct skip mark syntax

Author: Tracy-Walker

tests/security_and_privacy/test_private_browser_password_doorhanger.py

@@ -1,4 +1,4 @@
-from platform import system
+import sys
 
 import pytest
 from selenium.webdriver import Firefox
@@ -21,7 +21,9 @@ def add_prefs():
     return [("signon.rememberSignons", True)]
 
 
-@pytest.mark.skipif(system == "Linux", reason="136.0b3 Linux security bustage")
+@pytest.mark.skipif(
+    sys.platform.lower().startswith("linux"), reason="136.0b3 Linux security bustage"
+)
 def test_no_password_doorhanger_private_browsing(driver: Firefox):
     """
     C101670: Ensure no save password doorhanger shows up and settings are correct